Substance Use Disorder Workforce Loan Repayment Act of 2018 This bill amends the Public Health Service Act to create a loan repayment program for individuals who complete a period of service in a substance use disorder treatment job in a mental health professional shortage area or a county where the drug overdose death rate is higher than the national average. The substance use disorder treatment job must be a full-time position where the primary intent and function is the direct care of patients with or in recovery from a substance use disorder. Individuals must enter into an agreement of service of up to six years with the Health Resources and Services Administration. The repayment program shall pay one-sixth of the principal and interest on any eligible loan for each year of service; the maximum total amount of repayment by the program is $250,000 per individual.